The present invention relates, in general, to an imaging device for providing three-dimensional (3-D) image data and, more specifically, to an imaging device providing two-dimensional (2-D) image data with range data provided by laser time-of-flight (TOF) registered by individual pixels of an imaging array.
Various techniques are known for creating a three-dimensional image of a scene, i.e., a two-dimensional image which, in addition to indicating the lateral extent of objects in the scene, further indicates the relative or absolute distance of the objects, or portions thereof, from some reference point, such as the location of the camera.
At least three basic techniques are commonly used to create such images. In one technique, a laser or similar source of radiation is used to send a pulse to a particular point in the scene. The reflected pulse is detected and the time of flight of the pulse, divided by two, is used to estimate the distance of the point. To obtain the distance of various points in the scene, the source is made to scan the scene, sending a series of pulses to successive points of the scene.
In yet another technique, a phase shift, rather than time-of-flight, is measured and used to estimate distances. Here, too, the entire scene or relevant portions thereof must be scanned one point at a time.
In a third technique, which also involves scanning, at least a single radiation source and a corresponding detector are used, with suitable optics acting on the light in a manner which depends on the distance to the object, to determine the distance to a particular point in the scene using a triangulation technique.
The major disadvantage of all three of the above-described techniques is that each requires point by point or line by line scanning to determine the distance of the various objects in the scene. Such scanning significantly increases the frame time of the system, requires expensive scanning equipment and necessitates the use of fast and powerful computational means and complex programming.
To improve upon these techniques, some have attempted to eliminate the need for scanning objects in a scene by making 2-D imaging range finders, one example of which is the LAser Detection And Ranging (LADAR) technique. Using such technique, the number of laser pulses required is reduced, as all pixels integrate the reflected light simultaneously.
U.S. Pat. No. 6,091,905, which is incorporated herein by reference, describes a method for creating an image indicating distances to objects in a scene. The method includes modulating a laser source and detecting the received modulated signal with a detector. The method is based on the idea that a near object reflects light to the detector for a longer period of time during each cycle than a far object. The difference in duration of the detected reflected light during each cycle translates to a different intensity, or gray level, on the detector.
For example, assume that a point on object B is a shorter distance away from the laser source and/or detector than a point on object A. Then, reflected light from the point on B starts arriving at the detector relatively early in the active portion of the detector cycle and continues to be received by the detector until the detector is deactivated at the end of the active portion of the detector cycle. The reflected light from the point on B continues to proceed toward the detector for a period which corresponds to the period of irradiation. However, the portion of this reflected radiation which falls beyond the deactivation or blocking of the detector is not received by the detector and does not contribute toward the intensity sensed by the corresponding pixels of the detector.
By contrast, light reflected from the point on object A starts arriving at the detector later during the active portion of the detector cycle and also continues to be received by the detector until the detector is deactivated.
The result is that reflected light from a point on object B is received for a longer period of time than reflected light from a point on object A. As such, the intensity of gray level of each pixel during each cycle is related to the amount of time in each cycle during which radiation is received by that pixel. Hence, the intensity, or gray level, can be translated to the distance, relative or absolute, of the point on the object.
Another known 2-D imaging LADAR uses a constant current source at each pixel to charge a pixel capacitor while the pixel is illuminated. By measuring the integrated charge, a range measurement is read. This circuit suffers however, from pixel capacitor nonuniformities and temperature sensitivity.
The deficiencies of conventional devices used for providing 2-D image data and range data, based on reflected light registered by pixels of an imaging array, show that a need still exists for an improved device. The present invention addresses this need.
To meet this and other needs, and in view of its purposes, the present invention provides a three-dimensional imaging range finder using a transmitted pulse reflected from a target. The range finder includes a pixel sensor for receiving light from the target and the reflected pulse. A global counter is provided for determining a time-of-flight of the transmitted pulse. A processing circuit, which is coupled to the pixel sensor and the global counter, extracts the reflected pulse received by the pixel sensor, and stores the time-of-flight value upon extracting the reflected pulse. The pixel sensor provides a luminance signal and the processing circuit extracts the reflected pulse from the luminance signal.
In one embodiment, the processing circuit includes a matched filter for providing a peak triangular return signal centered on the reflected pulse. The processing circuit also includes a comparator for providing a write command, when the peak triangular return signal exceeds a predetermined threshold setting level and a memory for storing the time-of-flight value, when the write command is provided to the memory.
In another embodiment, the present invention provides a three-dimensional (3-D) imaging range finder using a transmitted pulse reflected from targets in a field-of-view. The 3-D imaging range finder includes a plurality of pixel sensors in an imaging array, each pixel sensor receives light, including the reflected pulse, from a respective target in the field-of-view. A global counter is included for counting predetermined intervals of time starting at transmission time of the pulse. A processing circuit is also included for extracting the reflected pulse from a respective pixel sensor and storing a value of the counted predetermined intervals of time, after extracting the reflected pulse. The global counter broadcasts the predetermined intervals of time to each of the pixel sensors in the imaging array. The imaging array provides a two-dimensional image of the targets in the field-of-view and the processing circuit of each respective pixel sensor provides depth information of the respective target in the field-of-view.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.